For several decades it has been known that the Drosophila neurogenic genes are required early in development for proper differentiation of the ectoderm. In mutant flies, cells that should form hypoderm (skin) instead differentiate as nervous tissue. Recently we have discovered a similar role for the neurogenic genes in mesodermal differentiation: Loss of any of the genes causes hypertrophy of certain cell groups in mesoderm. From sequence analysis and biochemical and cellular studies, the best characterized of the neurogenic genes, Notch, produces a large, unprocessed, transmembrane protein that is similar to mammalian clotting and growth factors. The structure of the protein suggests action in cell-cell communication in ectoderm and mesoderm. In this proposal we will extend an analysis of the neurogenic genes and their products as follows: 1) Embryos genetically mosaic for Notch will be examined to determine whether mesodermal development is linked to autonomous action of the gene in that germ layer, or inductive interactions between mesoderm and ectoderm. 2) We will search for hypotrophied cell groups in mesoderm of neurogenic mutants using new mesodermal cell markers. We will also search for a possible role for the neurogenic genes in endoderm development. 3) The effects of amino acid substitutions in the different EGF-elements of Notch will be assessed in transgenic Drosophila. 4) A map of Notch and Delta protein segments governing interaction of these molecules will be produced in vivo and in vitro. We will search for additional molecular targets for interaction with the Notch and Delta proteins. 5) A Drosophila homologue of the neuromodulin/neurogranin gene family will be characterized at the genetic, molecular and cellular levels.